The subject invention generally relates to circuits employing power switching transistors such as inverters, choppers and the like, and more particularly to improvements in snubber circuits for such power switching transistors.
The conventional method for controlling the load line of bipolar transistors is to connect a polarized snubber from the collector to the emitter as shown in FIG. 1. More specifically, (an NPN) transistor 11 is controlled by a base driver circuit 12 to alternately turn on and off so as to permit current to flow or block the flow of current through the load 13, (here represented as an inductor). The snubber circuit consists of a diode 14 and a capacitor 15 connected in series between a collector terminal and an emitter terminal of transistor 11 and a resistor 16 connected between the junction of a diode 14 and a capacitor 15 and the collector terminal of transistor 11. In this arrangement, a base driver power supply 17 (represented as a battery) must provide sufficient voltage to supply the base boost required to turn the transistor 11 on.
The basic purpose of the snubber circuit consisting of the diode 14, the capacitor 15 and the resistor 16 is to protect the transistor 11 at turn-off by transferring the collector current to the capacitor 15 via the diode 14 before the transistor voltage is allowed to rise. This reduces switching power losses in the transistor 11. The snubber capacitor 15 must be discharged prior to any subsequent turn-off of the transistor 11. This discharge is performed by the transistor 11 when it turns on through the resistor 16.
One problem with this snubbing technique is that it increases the switching losses during the period that the switching transistor is turning on, since the snubber presents a capacitive load to the transistor. The transistor must pick up both the load current and the snubber discharge current at turn-on. With a clamped inductive load, it must also pick up the reverse recovery current of a clamping diode 20.
A collector current waveform typically occurring in a push-pull circuit is illustrated in FIG. 2. The high peak collector current at turn-on requires more base current drive to maintain saturation. The base current drive requirement is further aggravated by the fact that most power transistor current gains (.beta.) drop off significantly at high collector current values. The peaking of the base drive current necessary to compensate for the low gain at high collector current is usually provided by a parallel RC circuit in series with the base terminal.